Republicans’ Senate Chances Rise Slightly to 60 Percent

For the last month, we’ve been adding one or two polls a day to The Upshot’s Senate forecasting model. Today, we update all 36 races, based on estimates from a YouGov online panel that covers every congressional and governor’s race across the country.

The panel, asked for its preferences in collaboration with CBS and The New York Times, is unusual in its scope: It comprises more than 66,000 people living in states with a Senate race this year. YouGov, a polling firm, also conducted online interviews with roughly half of the panelists in previous years, allowing it to know how they voted in the past.

With the addition of the YouGov estimates to our model, the overall outlook for the Senate remains roughly the same. The Republicans appear to have a slight advantage, with the most likely outcome being a Republican gain of six seats, the minimum they need to finish with a 51-to-49-seat majority. But we, like manyother forecasters, would not be surprised by a gain of anywhere from four to eight seats.

Summing up the possible outcomes, our model gives the Republicans a 60 percent chance of taking control, up from 54 percent on April 1.

Polls are only one part of the model. (And we adjust polls from partisan firms according to our best estimates of how Republican- or Democratic-leaning the pollster has been this cycle.) The model also includes the candidates’ political experience, fund-raising, a state’s past election results and national polling.

The relative weight of these factors depends on the number and the quality of the polls in each state, as well as how useful each factor was in predicting past Senate elections. Currently, polls make up about 80 percent of the forecast in the most competitive races.

The biggest effects of the YouGov estimates included movements toward the Republicans in Michigan, Georgia and North Carolina. These shifts were offset somewhat by positive numbers for Democrats in Colorado and Alaska.

In Michigan, Terri Lynn Land, the Republican candidate, leads Representative Gary Peters, the Democratic nominee, by nearly two percentage points, according to the panel. Before the panel result, Mr. Peters, whose House district includes parts of Detroit as well as Grosse Pointe, had led in all 10 of the polls in our database since mid-April. This is true even after we adjusted the polls for anticipated partisan bias or for having been conducted among registered, instead of likely, voters. (Democrats typically fare better in polls of registered voters.)

Even with the new data, our model gives Mr. Peters a slightly better than 50 percent chance to win. We’ll see over the coming weeks whether this movement holds. Much of the point of having a Senate model is to avoid reading too much into a single poll.

Something similar happened in North Carolina, which YouGov suggests is a tossup. We had begun to favor Senator Kay Hagan, a Democrat whose 2008 victory over Senator Elizabeth Dole was an upset. Because of the YouGov data, her chances have fallen, but our model still favors Ms. Hagan slightly, giving her just over a 60 percent chance to return to the Senate for a second term in January.

Another pro-Republican shift came in Georgia. Last week, David Perdue, a former chief executive of Dollar General stores, won a primary runoff to become the state’s Republican Senate nominee. We expected our poll-based average might move toward the Republicans, as it did in Iowa this year, once surveys moved past months of intraparty fighting, which can temporarily damage candidates.

But the model suggests it’s too early to write off Michelle Nunn, the Democratic nominee, and the state remains the best pickup opportunity for Democrats. The YouGov data, which is based on online interviews conducted over most of July, showed Mr. Perdue up by six points.

Some good news for Democrats came from Colorado, where Senator Mark Udall, a Democrat, was estimated to be leading by four points.

Further good news for Democrats came from Alaska, where Senator Mark Begich leads one of his possible Republican challengers, Dan Sullivan, by 12 points. The race has been sparsely polled — the previous poll in our database was from early May — and it is known for being a very difficult state to poll, so we regard these results cautiously. In addition, nearly a fifth of the panelists identifying as Republicans said they favored someone other than Mr. Begich or Mr. Sullivan in a two-way matchup. Joe Miller, the Tea Party favorite who beat Senator Lisa Murkowski in the Republican primary in 2010, remains something of a wild card in the race.

The chart below shows how the YouGov panel affected our model in nine of the closest Senate races. To gain control, Republicans probably need to win at least five of the races below.

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A version of this article appears in print on , on Page A10 of the New York edition with the headline: G.O.P.’s Senate Chances Rise Slightly, Data Show. Order Reprints | Today’s Paper | Subscribe